Shelf



FIG. 1 is an isometric view of a shelf embodying the design of my invention showing the front, the top and the left side thereof;

FIG. 2 is a side elevational view taken from the right side of FIG. 1;

FIG. 3 is a bottom plan view thereof; and

FIG. 4 is a rear elevational view thereof. 

The ornamental design for a shelf, substantially as shown and described. 